Hello all, I am working on a 1990 CBR 1000F that has been restored to a beautiful condition. I am not sure about all the things that have been done in the past, but we are finding details along the way. The last shop that worked on it said that it was running great when it left their shop...

The bike has English Motad Slip-Ons, stock air box, Dyno Jet stage one jetting kit. Everything else seems stock.

We have done an in depth carb clean, new plugs and multiple checks as per the shop manual. I do still need to do a compression test. Everything checks out as in working order and correct.

Motad claims that the slip-ons have identical back pressures to the stock cans and do not require re-jetting. Dyno-Jet claims that the stage one kit is for a stock bike and mearly fixes the toned down settings that allow import. The jet kit is installed per directions.

The problem is that the bike runs so rich that it runs everybody out of the shop and makes your eyes water. After it is at operating temperature, a small bump of the throttle sends the RPMs strait up to 4K. It will stay there until you put a load on the engine and bring the RPMs down. Then it reverts to idleing normally. Another bump of the throttle yields the same 4 grand. Another clue is that the choke will send the RPMs sky high.

There are two possible problems that I have yet to remedy. The air box, where the individual tubes attach has possible leaks. There is a black goo sealing the joint and some of that is cracked. I am assuming this would not show the problematic symptoms we are seeing (rich mixture).

The other possible is what I need advise on. The vent tubes coming from the vacuum actuated needle diaphrams goes to a small filter box. That filter is non-existant and it looks like someone had drilled four extra holes in it. I cannot tell from the parts diagram if there it is supposed to have one large hole and four smaller ones. Is this a trick to have the RPMs come back to idle faster. It seems feasable that this is our problem.

Any help would be welcomed eagerly. Thank you for your expertise.

 

The hanging high RPM 4000, until loaded may be the carbs not in sync. Hook up some sticks or gauges and balance them up. This may also help find a vacuum leak or clogged air passage on one carb. Air leaks upstream of the carbs are not an issue generally. I would start with retuning to stock jetting also. Even it's lean it won't hurt it to idle and run in mid ranges. I can't speak to the vacuum hose/filter box issue, not on my model. As long as they are all the same, ie, none of them are clogged/pinched off-should be ok.

 

For what it's worth, I've got a full V&H (very free flowing )4>1, K&N element, and I found the Dyno-Jet still very rich, so I can't see how it'd be right for a stock back pressure system. I ended up with the smallest main jets in and the needles on their leanest position. I also ditched the springs as there was a massive flat spot due to the slides going up way too fast.

Maybe the slides are sticking due to the p!ss weak springs. I'd try some stock springs and set the needles on their leanest and go from there. Sorry not to be more help but I'm sure somebody else will have some more ideas.



Leaks in the air intake would have to make it leaner and run rough, so I can't see that causing it.

 

Thank you both so much... That makes complete sense... Will order all the stock parts... Thanks again... Be back with what I find...

 

Just to confirm- Motads certainly run at a very similar back pressure and flow to stock- and will run well on stock fuelling

I think the good advice you have already have will sort it!

 

Hello again, I have now managed to get all the parts together to put the carbs back to the stock jetting, except... the needle return springs. Honda has discontinued them and we have been led down the long path to finding this out twice. Very frustrated about these springs. Wondering if anyone on the forum has these stock springs for a 90 CBR1000F.
The bike is running a lot better with the stock jets and needle back in. It does still stick at 4K rpms. Putting everything back together now and desperatly trying to find these vaccum return springs for the needle slide. Any leads would be greatly appreciated. Thanks again.

 

Sounds like your piston is sticking in one carb...............

 

Hello, we have finally located some springs! Slapped them in and voila. Now a carb balance should smooth it right out. Thanks for all ya'lls help. Love this bike. Very smooth...
It seems that there is a rare chance that Honda will all of the sudden have stock for parts they have long discontinued. I assume this comes from old stock at a dealership that goes under. Try, try again.

 

That is good news